Process for stripping color from dyed wool



April 17, 1951 E. R. KASWELL PROCESS FOR STRIPPING COLOR FROM DYED WOOL Filed July 14, 194'? JNVENTOR. ERNEST R Kq SWELL BY HTTOR/VEV wool and the like.

Patented Apr. 17, 1951 PROCESS FOR STRIPPING COLOR FROM DYED WOOL Ernest R. Kaswell, Boston, Mass., assignor to Fabric Research Laboratories, Inc., Boston, Mass., a corporation of Massachusetts Application July 14, 1947, Serial No. 760,765

zinc or sodium formaldehyde sulfoxylate, sodium hydrosulphite, and similar strong reducing agents. These stripping solutions are used com-- mercially at or about their boiling temperatures and their action on the wool is so drastic that the hazard is present that cystine linkages oi the wool may be broken or the chemical or physical nature of the wool otherwise fundamentally al tered. In addition, the formaldehyde content of the suifoxylate-formaldehyde stripping agents usually exerts a tanning action on the wool, resulting in a harsh hand. Further, some of these agents impart an objectionable odor to the product. As practiced commercially, stripping impairs the hand and tensile strength of the wool to a greater or less extent depending upon the temperature and the duration of the stripping treatment, which in turn depends upon the shade of the color and the nature of the dyestuif to be stripped. Titanous sulfate, also, is employed and will answer in so far as color removal is concerned, but the product of the stripping reaction,

titanic sulfate, is unstable and hydrolyzes to form titanium dioxide, a White insoluble pigment. Ihis pigment is precipitated on and in the wool fibers, making them harsh and brittle, as well as discolored. With all of these prior stripping agents the reducin agent concentration decreases during the course of the strip, so that excessive concentrations, which may be injurious to the product, must be employed at the start, or additions of stripping agent made from time to time or the wool exposed to the stripping agent for an unduly long time, resulting in imparting to the wool undesirable characteristics such as a harsh hand or loss of tensile strength:

The present invention has an object to provide an improved process for stripping colors from A further object is to provide a stripping process that is much more eilicient in removing color and which will strip some colors not capable of being stripped with presently available stripping processes. A further object is to provide a stripping process which does not significantly affect the hand or strength of the wool.

. In my process the colors are stripped from wool and the like .by solutions containing vanadium,

2 titanium or chromium ions in a low valence state While the concentration of ions of this valence state is continuously or substantially continuously maintained during the treatment by electrolysis of the solution. I prefer that the solutions should contain at least some ions in their lowest valence state.

The metallic ions are reduced to the required low valence state by employment of an electrolytic cell containing an appropriate metallic salt or salts wherein electrolytic reduction takes place on one side of a porous diaphragm and oxidation on the other. The current may be passed through a solution which is in contact with the wool, or it may be passed through the solution before it is brought into contact with the wool, if the solution is thereafter brought into contact with the wool without any substantial intervening exposure to the atmosphere or other oxidizing medium.

It appears that the stripping action of the metallic ions is due to their reducing action on the dye, in the course of which reduction the ions are oxidized to a higher valence state. The ions so oxidized are again reduced by the current to a lower valence state, and the oxidation-reduction cycle is repeated. The reducing or stripping power of the solution is maintained at a more or less constant level instead of falling off steadily as in the prior commercial stripping processes referred to above. The metallic ions are not consumed during the treatment.

My process is applicable generally to dyed animal fibers, i. e., keratinous material such as wool, fur, hair, etc, typified by wool. It will be further described herein by reference to wool, by which term I mean to include all of these keratinous materials.

My process will strip colors which have heretofore been extremely difficult or impossible to strip with any of the processes heretofore known. Wool so stripped may be satisfactorily redyed with light colors in the usual manner. The process employs as its raw material dry re-agents, thus avoiding the shipping and storing of bulky liquids. as has been required in the past. It is unnecessary to subject the wool to boiling temperatures in the stripping solution and the hand and strength of the wool is not significantly affected by the process of this invention.

The process may be carried out as a batch 'PI'OCBSs within an electrolytic cell by placing in the cell a stripping medium comprising a soluble vanadium, titanium, or chromium salt, or a mixture of such salts, passing an electric current through the cell to reduce the metallic ions to a low valence state and then immersing the wool in the cell with continued passage of the current.

I prefer, however, to strip the wool in a circulating medium. This may be done by exposing the wool to a continuously circulated body of stripping medium which is reduced in a separate regenerating zone, for example in an electrolytic cell, and then is circulated to a stripping zone, for example, a tank or other vessel, in which the wool is immersed in the circulated liquid medium. Spent stripping liquor from the stripping zone may be returned continuously to the regenerating zone where the oxidized vanadium, or other metallic ions are again reduced electrolytically to the active state.

By so conducting the stripping, mechanical interference with the electrodes by the wool which is undergoing processing is avoided. The objectionable hydrogen and oxygen gases liberated from the electrolytic cell can be disposed of more readily, and this cell may, for example, be located in a separate room. Also, the design of the cell is not limited by the need to provide a space for the reception of the wool and a more eificient and compact cell thus may be employed.

Further, better circulation of the liquor through the wool is obtained and rinsing of stripping liquor'from the wool at the end of the strip is facilitated.

An illustrative form of apparatus by which my invention may be practiced is shown in the accompanying diagrammatic drawing, wherein the single figure is a schematic view showing the layout of the apparatus and the construction of an electrolytic cell.

Referring to the drawing, an electrolytic cell, Within which the stripping solution mav be reduced electrolytically, is shown at 2. Piping is provided for conducting stri ping liquor from the cell 2 to a stripping vessel 4 and for returning spent liquor from the vessel 4 to the cell 2. P ovision is made for adding make-up liquor or withdrawing liquor from the syst m, as required, and for rinsing the stripped wool without removing it from the tank 4.

The electrolytic cell 2 comprises an outer shell 6. Within the shell 6 is a porous not 8, o en at its upper end. An anode IU of sheet tin or lead in the form of 'a cylinder open at both ends is disposed within the cell on the out ide of the porous pot 8 and surrounding its cylindrical wall. A cathode l2, likewise of tin or lead, is disposed inside the not closely adjacent its c lindrical wall. This cathodemay conveniently take the form of an open-ended cylinder and should be of a porous construction, for example, of a wire gauze or a perforated solid metal sheet. 'The cathode and the anode are respectively connected to a suitable source of direct current through leads l4 and I6.

The reduced strirmine liquor is withdrawn from the bottom of the electrolytic cell 2 by means of a pump l8 through a valve 20 and pump intake line'22. The'pump l8 discharges toa line 24 which leads into the bottom of the stripping tank 4. A valve 26 in line 24 permits adjustment of the rate of supply of stripping liquor to tank 4. The stripping tank 4 is provided with means for permitting the wool which is to be stripped to 'be placed in and removed from the tank. As showmthe tank is provided with a cover 23 which may be clamped in closed position by means of pivoted clamps 3!! which engage in ears 32 on the edge of the cover 28. The inner surface of the 4 tank should be inert to the stripping medium and the tank may for this purpose be made of an inert material or provided with a liner of an appropriate resistant material. The tank is equipped with a closed steam coil 34 for heating its contents.

Spent stripping liquor is discharged at the top of the tank 4 through a line 36 by which is passes,

through valve 38 to a reservoir 40.

The reservoir 40, discharges into the top of the cathode portion of the electrolytic cell 2 through a line 42 containing the valve 44. The system may be originally charged with stripping medium through a line 46 leading into the top of the electrolytic cell through a valve 48.

If the conditions of operation are such that the capacity of theelectrolytic cell is more than adequate to regenerate the stripping medium at the rate of new through the system required for the work in hand, then a part of the liquor discharged from reservoir 40 may be bypassed around the electrolytic cell and returned directly to the stripping tank. For this purpose, a bypass line 50 is provided leading from the bottom of the reservoir 40 through a valve 52 to the pump intake line 22.

A branch line 53 containing valve 54, connects the discharge of the pump [8 directly to the reservoir 40.

When the process has been carried to the point where the color is sufiiciently stripped from the wool, the tank 4 may be emptied of stripping liquor and the wool may be rinsed in the stripping tank 4. For this purpose a line 56 is provided leading from the bottom of the stripping tank through a valve 58 to the pump intake line 22. When the wool has been stripped, the valves 54, 58 and 66 may be opened and the valves 26, 20, 52 and 38 closed so that pump l8 pumps liquor from the stripping tank through line 56 and into the reservoir 40 through line 53. When the tank 4 has been emptied, the pump I 8 may be shut down and rinse water circulated through the wool in the stripping tank 4 by introducing it through rinse water supply line Wand valve 62 and discharging it through a drain line 64,'containing valve 66.

A r may be vented from the system through valve 68 in the top of the reservoir 40. The hydrogen and oxygen liberated in the cell are individually drawn off by suitable piping not shown.

The process may be conducted by employing an electrolytic cell similar to the one shown at 2 and introducing the wool directly inside the porous pot. In this case the removable cover 61 of cell 2 will be utilized for the introduction and removal of the wool. The cathode may be provided with a series of openings, shaped as shown at 69, to direct gas bubbles formed in the space between the cathode and the inner wall of the pot 8 inwardly of the cathode, and at the same time pull freshly reduced solution inwardly of the cathode l2 and thus into contact with the wool. The openings 69 thus create circulation of the liquor within the cell 2 with accompanying increase of the rates of reduction and stripping. Cell 2 also may be provided with appropriate inlet and outlet piping connections for introducing and withdrawing the stripping solution and for circulating rinse water through the cell. Means for venting oxygen and hydrogen and means for introducing carbon dioxide or other inert gas into the cell to purge its headspace of air and protect the stripping liquor from oxidation may be provided. Means 'for mechaniigUlDg-UBQ- 51 cally agitating the wool. also: maybe provided. The-system shown maybe modified by the in.- troduction of automatic" gas vent valves, pres sureregulating valves, or other-refinements without departure from the invention. The apparatus may be operated either'continuously 'orintermittently.

It will be understood that the apparatus arrangement shown schematically in the drawing and described above represents only one illustrative arrangement of apparatusin which my process may be conducted. It isessential only that Color Index No. 1060 (sodiumsalts of a complex the stripping liquor be protected from oxidation Vanadium pentoxide gms 2.0 Sulphuric acid, sp. gr. 1.84 ml. 32.0 Water ml. 1000 The vanadium pentoxide and sulphuric acid were mixed, the mixture heated and stirred to form a clear yellow to orange solution, and then poured into the water. The apparatus was then charged with suffic ent of this liquid to fill the tially fill the reservoir. The electriccurrent was turned on and adjusted to a current flow of about 0.01 ampere per square centimeter of cathode 1 area and the temperatureof' the liquid in the strippin tank rai ed to and held within the ran e of 85 to 95C.

Upon imposition of the current, oxygen was evolved at the positive electrode and hydrogen was evolved at the negative electrode. It was observed that the vanadic sulfate was progressively reduced through the green vanadium valence 3 state to the lavender valence 2 state. Re-

duction of the vanadium to the lavender (-ous) state mav be accomplished in about one hour when the li uid is not in circulation. Preferably, the pump I8 is not placed in o eration until the liqu d in the cell is fully or substantially reduced.

When the stri ping liquor had reachedthe -ous state as shown by its color (or as may be determined by measurement of the ela sed time) the strip ing tank 4 was charged with a batch of wool flannel dyed with Alizarin Blue S E, Color Index No. 1053 (sodium salt of 4,8'-diamino-l,5-clihydroxy-anthraouinone-2-sulphonic acid) and the cover 28 closed and clamped in place. The temperature in the stripping tank was held in the range 85 to 95- C. The pump I 8 was adjusted to cause the liquor to flow through tank 4 at such a rate as to give good circulation of the stripping liquor through the wool. After minutes, the wool was rinsed with tap water, cover 28 opened and the wool removed. The wool was found to be stripped to alight natural shade. Its handle was soft and comparable with that of the wool before stripping.

Abatch of indigo-dyed, 11 .oz..wool flannel having a tensile strength of 18 pounds per inch. of width was stripped by the same procedure and stripping agent. The stripped wool was very light tan in color and showed a tensile strength of 16 pounds per inch of width. There wasno loss in weight.

Similar. stripping runs were made with wool electrolytic cell and the stripping tank and par- Water 1500 m1.

mixture of isomeric. diaminq-dihydroxy-anthraquinone sulfonic acids); Alizarin Red :8, Color Index No. 1034 (sodium salt of 1,2-dihydroxyanthraquinone-3-sulphonic acid; with Eriochrome Black T, Color Index No. 203 (Sodium salt of 4-sulpho-5-nitro-2-hydroxy-a-naphthalene-azo-a-naphtholl; with indigo, and with indigo on an Alizarin Red S base. In each instance thecolor was effectively stripped without significant impairment of the handle or the tensile strength of the fabric. I In my process I can use titanium sulfate as a stripping solution without the disadvantage of deposit of titanium dioxide on the wool fibres. This is so because the continuous electrolytic reduction of the stripping solution keeps the concentration of titanic sulfate, which hydrolyzes to titanium dioxide, at a minimum.

Examples of titanium sulfate stripping solutions suitable for use in my process are the following: 1

Titanium sulfate 4.4 gms. 13.2 gms. Sulfuric acid (sp. gr. 1.84) 32 ml. 32 ml. Water 1000 ml. 1000 ml.

The stripping liquor is made by mixing together the titanium sulfate and the acid, heating to solution and then stirring the product into the Water.

Examples of suitable chromium stripping solutions are the following:

Chromic anhydride, CrO3 1.0 gms. 0.20 gins. Sulfuric acid 30.0 ml. 20ml.

Mixtures of the various stripping solutions referred to above may be used.

I have found that the process may be somewhat shortened and the color somewhat improved if the stripping liquor contains a wetting agent. I may use for this purpose any wetting agent which is not adversely affected by the conditions in the stripping system such as by the conditions in the electrolytic cell. Examples of suitable Wetting agents, are Z-methyl 2,4-pentanediol, the butyl ether of ethylene glycol and ALKaterge C, a non-volatile, cationic amine type of surface active agent. The amounts of these Wetting agents to be used are not critical. Thus, the stripping solutions described above may contain about 1 ml. of ALKaterge C dissolved in 100 ml. of butyl ether of ethylene glycol, or may contain 20-30 ml. of 2-methyl-2,4- pentanediol.

The preferred current density may vary. I have obtained excellent results with current densities within the range 0.0087 to 0,012 ampere per square centimeter.

The stripping time may vary from 5 minutes to one hour, dependin upon the color being stripped and degree of stripping desired.

Generally, temperatures above 75 C. and below the boiling point of the stripping liquor produce good results. Temperatures inthe range C. to C. are preferred.

As the material for the electrodes, I presently prefer to use tin or lead. These metals give good, uniform results and no trouble arises as a result of poisoning of the electrodes during the stripping operation. Platinum is also satisfactory for the electrodes but it is expensive and is subject to the disadvantagethat in the course of a run the electrodes lose their efliciency through cbntamination' 'or poisoning resulting from the production of 'free sulfur in the bath by the reducin action of the vanadous sulfate on the sulphuric acid or from contamination from the reduced dye or the wool or some other source. A suitable cathode may also be made of graphite, or of Duriron, an iron-silicon alloy.

I claim:

1. The process for stripping color from keratinous material which comprises treating the keratinous material with an electrically-conducting acid liquid stripping solution containing 8 in "their lowest valence state ata "temperature above 75 C. and below the boiling point of the liquid, whereby color in the wool' is reduced and stripped by the reducing action ofJs'aid low valence ions with accompanying oxidation of said ions to a higher valence state, reducing the va- "state, immersing the colored keratinous material to be stripped in'said stripping medium in a stripping zone wherein color in the keratinous material is reduced and stripped by the reducing thereafter contacting the keratinous material with said-solution. V

2. The process for stripping color from keratinous material which comprises treating the keratinous material with an electrically-conducting acid liquid stripping solution containing metallic ions selected fro'm'the group consisting tion, and thereafter contacting the keratinous material with said solution The process for stripping color from wool which comprises'treating the wool with an electrically-conducting acid liquid stripping solution containing a wetting agent and metallic ions selected from the group consisting of Vanadium, titanium and chromium ions in their lowest valence state, whereby color in the wool is reduced and stripped by the reducing action of said low-valence ions with accompanying oxidation of said ions to. a higher valence state, reducing the valence state of ions so oxidized by electrolysis of said solution, and thereafter'con- .tacting the wool with said,solution.-- i

4. The process for stripping color from wool which comprises immersing the wool in an electrically-conducting acid liquid stripping solution contain ng vanadium ions in their lowest valence state, whereby color in the wool is reduced and stripped by the reducing action of said vanadium ions with accompanying oxidation of said vanadium ions to a higher valence state, reducing the valence state of vanadium ions soox dized by electrolysis of said solution, and thereafter contacting the wool with said solution; 1 -5. The process for stripping color from wool which'comprises immersing the wool inan electrically-conducting acid liquid stripping solution containing chromium ions in their lowest valence state, whereby color in the wool is reduced and 'stri'ppedbythe reducing action of said chromium ions with accompanying oxidation of said chromium ions to a higher valence state-"reducing the valence state of chromium ions so oxidized by electrolysis of said solution, and there'- after contacting the wool with said solution.

6. The process for stripping color from wooi which comprises treating the wool with an elect rically-conducting.acid liquid stripping medium ;c9ntainmg a wetting. agent and vanadium-ions of vanadium, titanium and chromium ions in action of said lowvalence ions withaccompanying oxidation of said ions to a higher'valence state, circulating stripping medium from said stripping zone to an electrolytic cell for regeneration, reducing said oxidized ions'to a low valence state in the cathodic compartment of said cell by electrolysis of said medium, andcirculating stripping medium containing ions so reduced from said cathodic compartment to said stripping zone.

,8; The process for stripping color from keratinous material which includes providing an acid liquid stripping medium containing vanadium ions in a low valence state, immersing the colored' keratinous material to be stripped in said stripping medium in a stripping zone wherein color in the keratinous material is reduced andstripped-bythe reducing action of said vanadium ions with'accompanying oxidation of said ions to a higher valence state, circulating stripping medium from said stripping zone to an electrolytic cell for regeneration, reducing said oxidized-'vanadium'ions to a low valence state in stripping zone:- 9. The process for stripping color from-kerat inous material which includes providing an acid liquid stripping medium containing a wetting agent and vanadium ions in a low valence state,

immersing the colored keratinous material to be stripped in said stripping medium in a stripping zone wherein color in the keratinous material is reduced and stripped by the reducing action of said vanadium ions with accompanying oxidaltion of said ions to a higher valence state, circulating stripping medium'from said stripping zone to an electrolytic cell for regeneration, reducing said oxidized vanadium ions to a low valence state 7 in the cathodic compartment of said cell byielectrolysis of said, medium, and circulating stripping medium containing vanadium ions so reduced from said cathodic compartment to said stripping zone.

10, The process for stripping color from keratinous material which includes providing an acid liquid' stripping medium containing a wetting agent andchromium ions in a low. valence state,

f 'imrne'rsi'ngfthe colored keratinous material to be stripped in said stripping medium in a stripping zone wherein color in the keratinous material is reduced and stripped by the reducing action of said chromium ions with accompanying oxidation of said ions to a higher valence state, cir- Clllating stripping medium iron; .saidstripping zone 'toan electrolytic' -ce1l for regeneration, re-

ducing said oxidized chromium ions to a low valence state in the cathodic compartment of said cell by electrolysis of said medium, and circulating stripping medium containing chromium ions so reduced from said cathodic compartment to said stripping zone.

11. The process for stripping color from keratinous material which comprises treating the keratinous material in a stripping zone with an electrically-conducting acid liquid Stripping me dium containing metallic ions selected from the group consisting of vanadium, titanium and chromium ions in a low-valence state, whereby color in the keratinous material is reduced and stripped by the reducing action of said low valence ions with accompanying oxidation of said ions to a higher valence state, reducing the valence state of so oxidized ions by electrolysis of said solution at a faster rate than the rate at which ions are oxidized, and thereafter contacting the keratinous material with the solution, whereby the concentration of said low valence ions in said stripping zone is maintained substantially constant during said stripping treatment.

12. The process for stripping color from W001 which comprises immersing the Wool in one zone of an electrically-conducting acid liquid contain ing chromium ions in their lowest valence state, whereby color in thewool is reduced and stripped by the reducing action of said chromium ions with accompanying oxidation of said chromium ions to a higher valence state, simultaneously re-.

ducing the valence state of chromium ions so oxidized by electrolysis of said liquid by an electric current passing between a cathode immersed in said liquid in said zone thereof and an anode immersed in said liquid in another zone thereof, and restricting flow of said liquid between said two zones.

ERNEST R. KASWEIL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 537,403 Burton Apr. 9, 1895 1,055,504 Albrecht 1 Mar. 11, 1913 2,073,664 Weisberg et a1 Mar. 16, 1937 2,307,137 Kennedy Jan. 5, 1943 OTHER REFERENCES Kaswell et al., American Dyestufi Reporter, Feb. 23, 1948, p 119430.

Jowett, Journal Society Dyers and Colourists, v01. 60, N0. 8, August 1944, DD. 197-200. 

1. THE PROCESS FOR STRIPPING COLOR FROM KERATINOUS MATERIAL WHICH COMPRISES TREATING THE KERATINOUS MATERIAL WITH AN ELECTRICALLY-CONDUCTING ACID LIQUOR STRIPPING SOLUTION CONTAINING METALLIC IONS SELECTED FROM THE GROUP CONSISTING OF VANADIUM, TITANIUM AND CHROMIUM IONS IN A LOW VALENCE STATE, WHEREBY COLOR IN A KERATINOUS MATERIAL IS REDUCED AND STRIPPED BY THE REDUCING ACTION OF SAID LOW-VALENCE IONS WITH ACCOMPANYING OXIDATION OF SAID IONS TO A HIGHER VALENCE STATE, REDUCING THE VALENCE STATE OF IONS SO OXIDIZED BY ELECTROLYSIS OF SAID SOLUTION, AND THEREAFTER CONTACTING THE KERATINOUS MATERIAL WITH SAID SOLUTION. 